For many applications it is desirable and, in fact, necessary to provide A.C. electrical power with a relatively constant voltage and frequency and without any interruptions. Hospitals, electronic data processing installations, radio and television stations, police communication facilities, airport communication and control tower directing facilities, and military installations are examples of electrical users that are critically affected when their A.C. power lines fail or when the voltage and/or frequency drop below or increase above certain relatively narrow limits. Their electrical equipment can misfunction or malfunction when any of these power line faults occur, and serious loss or damage may take place, and in some cases, lives may be jeopardized.
While public utility power systems are regarded as being quite reliable, they do have problems comprising electrical irregularities and failures. Public utility A.C. power lines are subject to numerous brief electrical surges, dips, transients and other faults, while major longer time faults and interruptions occur though less frequently and prolonged complete power failures arise occasionally. Details of such power system faults are given in the following literature: "Focus on Uninterruptible Power Supplies and Inverters" appearing on pages 100 to 107 of the May 10, 1975 issue of "Electronic Design"; "Power Protection Equipment" on pages 38 to 43 of the July 1977 issue of "Mini-Micro Systems" magazine; and "There's a Time and Place for UPS" on pages 39, 42, 43, 46 and 47 of the August 1979 issue of "Infosystems".
Briefly, in the period of a year, on an average there may be thousands of electrical irregularities such as voltage surfaces, dips, and transients lasting only a few milliseconds in any given power line. Annually, there may be hundreds of voltage and frequency drops lasting a few cycles and not exceeding about a 25% voltage drop, and up to about 10 complete outages or major line failures where the voltage drops 75% or more, with each outage lasting not more than about 15 minutes or so. Excluding deliberate blackouts or brownouts by the public utility operators in cases of excessive power demands, at rare intervals of perhaps one every year or two on the average, the damaging results of hurricanes, major sleet and wind storms, accidents in or to substations and power plants, fires and so on, can cause utility line interruptions lasting for hours or days.
It would be highly desirable to isolate or insulate loads sensitive to power line faults, irregularities and brief interruption lasting up to about 15 minutes, so that a steady input of electrical A.C. power at a selected voltage and frequency can be supplied regardless of what happens to the utility power line. Desirably, the equipment for so isolating or insulating the load from the power line should be reliable, efficient, of relatively low cost, require small space and need little maintenance, and should be effective to furnish the A.C. power for a period of time of the order of up to 20 minutes when the power line is completely interrupted. This will take care of practically all the normally expected power line problems. For critical facilities where standby generators with an engine are provided, these can be started easily in such to furnish power for the rare instances of hour or day long power outages. The present invention is provided to meet these parameters.